This invention relates to a model-based shift control for an automatic transmission, and more particularly to a pressure control of the on-coming clutch during a garage shift.
In general, a motor vehicle automatic transmission includes a number of gear elements and selectively engageable friction elements (referred to herein as clutches) that are controlled to establish one of several forward speed ratios between the transmission input and output shafts. The input shaft is coupled to the vehicle engine through a fluid coupling such as a torque converter, and the output shaft is coupled to the vehicle drive wheels through a differential gearset.
While most transmission shifts involve coordinated disengagement of an off-going clutch and engagement an on-coming clutch, the class of shifts commonly referred to as garage shifts (neutral-to-forward, neutral-to-reverse, forward-to-reverse, reverse-to-forward) ordinarily involve simply engaging an on-coming clutch, or disengaging an off-going clutch and then engaging an oncoming clutch. A representative control for garage shifting is set forth in the U.S. Pat. No. 5,046,178 to Hibner et al., issued on Sep. 3, 1991, and assigned to the assignee of the present invention. In Hibner et al., the oncoming clutch is filled in preparation for engagement, and when a transmission input speed disturbance indicative of on-coming clutch torque capacity is detected, a closed-loop control of the on-coming pressure forces the input speed to follow a desired speed profile until the synchronous speed of the target speed ratio is reached, whereupon the shift is completed by fully engaging the on-coming clutch.
While the above-described control is capable of achieving reasonably good garage shifting under controlled conditions, variations in the transmission input torque (due to changes in engine output torque and/or the torque ratio of the torque converter) can significantly degrade the shift quality. Additionally, variations in mechanical and/or hydraulic stiffness (compliance) from transmission-to-transmission tend to result in vehicle-to-vehicle shift quality variations. Accordingly, what is needed is a control that produces consistent high quality garage shifts in spite of these variations.
The present invention is directed to an improved control for an automatic transmission garage shift, wherein a dynamic model of the transmission is used to estimate the transmission input torque during the shift and to schedule the on-coming clutch pressure in accordance with the estimated input torque to achieve a desired input shaft trajectory. The transmission input torque is estimated based on two different methodologiesxe2x80x94one suited to steady-state engine idle conditions, and the other suited to engine output torque transient conditions. A fuzzy summation of the input torques provided by the two methodologies is utilized in transitions between the two conditions. Shift quality variations due to variations in mechanical and/or hydraulic stiffness are minimized by a pause inserted between the end of the fill period and the initiation of on-coming clutch pressure control, which results in a reasonably consistent degree of perceived transmission-to-transmission hydraulic and mechanical compliance. As a result, the control of this invention provides consistent high quality garage shifting, with less intensive calibration effort and improved adaptability to different powertrain and vehicle-type configurations.